1. Field of the Industrial Application
This invention relates to a rolling bearing which is used in boundary lubrication state, being long in service life.
2. Description of the Prior Art
An oil film parameter .LAMBDA. is employed as an index concerning the service life of a rolling bearing. The parameter .LAMBDA. represents the degree of formation of an oil film which greatly affects the lubrication of the rolling bearing (cf. "Rolling Bearing Engineering" edited by Rolling Bearing Engineering Editing Committee, and published by Yokendo, Jul. 10, 1975, first edition). The parameter .LAMBDA. is represented by the following equation: ##EQU1## where hmin=minimum oil film thickness hr.sub.1 and hr.sub.2 =square average roughnesses of two contact surfaces (the rolling contact surface of a bearing ring and rolling elements)
As is apparent from the above-described equation, as the value of the parameter .LAMBDA. increases, the oil film thickness is increased, and accordingly the life of the rolling bearing is increased. However, it should be noted that when .LAMBDA..ltoreq.1.5, the rolling bearing is placed in so-called "boundary lubrication state", and its service life is decreased.
Hence, it has been considered that, in order to increase the service life of a rolling bearing, it is essential to machine the rolling contact surfaces of each of the rolling elements and the bearing ring as smooth as possible and to sufficiently form oil films in the rolling bearing.
However, to do so is rather difficult depending on the structure of the rolling bearing. For instance in the case where a needle roller bearing is used in such a manner that the inner race is omitted, and instead the shaft is used as a bearing surface, it is considerably difficult to machine the bearing surface accurately until its surface roughness becomes substantially equal to that of the needle rollers. Therefore, if such a rolling bearing is used under operating conditions that the two contact surfaces are liable to be directly brought into contact with each other, then the rough surface of the shaft is brought in contact with the smooth surfaces of the needle rollers, as a result of which the needle rollers are slightly peeled; that is, they suffer from so-called "peeling damage".
In order to eliminate the above-described difficulty, peeling damage, Japanese Patent Application Publication No. 30008/1989 has proposed the following method: That is, in the method, the surface roughness of the rolling surfaces of rolling elements rolling on a shaft having a rough surface is in a range of 0.3 to 1.5 .mu.mRmax represented by maximum height, and a residual stress layer of 500 MPa or more is formed in the surface layer of each rolling element.
The machining conditions provided by the method are for the rolling elements only; that is, the method does not take into consideration the relationships between the surface roughness of the shaft, on which the rolling elements roll, and the surface roughness of the rolling elements. Hence, in the case of a rolling bearing in which the surface roughness of the rolling elements in combination with the surface roughness of the shaft does not satisfy certain conditions, the method is not so effective in preventing the rolling elements from the peeling damage.